The invention relates to a method for controlling a composition of a fuel/air mixture supplied to an internal combustion engine, in particular, an internal combustion engine embodied as a two-stroke engine. The internal combustion engine has a cylinder with a combustion chamber provided with a spark plug. The combustion chamber is delimited by a piston driving a crankshaft. An ignition control unit is connected to the spark plug and, depending on the rotary speed of the crankshaft, triggers at an adjusted ignition timing an ignition spark at the spark plug. A speed control circuit maintains the actual rotary speed of the crankshaft within preset limits. A controllable metering device is provided for at least one component of the fuel/air mixture.
Hand-held power tools such as motor chain saws, trimmers, blowers or similar devices are preferably driven by an internal combustion engine, for example, a two-stroke engine, that, when employing modern technology, is lively, powerful, and still can be operated in an environment-friendly way. In order to optimize operation of such a small-size engine with a displacement between approximately 20 cc and approximately 250 cc (cubic centimeters), microprocessors are used for engine control that, depending on operating parameters, control ignition of the internal combustion engine.
In regard to fuel systems it is also known to employ electromagnetic (solenoid) valves as disclosed in U.S. 2005/0168310 A1. Such a valve is used for controlling the fuel flow in order to adjust the quantity of fuel in regard to different operating parameters of the internal combustion engine.
When the engine speed of the internal combustion engine is monitored by a speed control circuit, the electronic system intervenes by changing the ignition timing in order to maintain the desired engine speed. In this way, the set idling speed during idling is controlled also. The controller used in this connection is usually a so-called PI or PID controller, i.e., a controller with a proportional term and an integral term.
Particularly in the case of a two-stroke engine, the engine speed is not only dependent on the ignition timing but also on the composition of the mixture. In case the mixture can also be varied as a control variable, a control intervention in regard to the mixture composition, on the one hand, and in regard to ignition timing, on the other hand, in particular during idling, can cause the system to undershoot or overshoot the control range. If this is the case, the engine can stall or the idle speed can no longer be maintained.